This invention relates primarily to low NOX combustion of gaseous fuels and more particularly concerns combustion in compact boilers and liquid phase heaters employing flue gas recirculation (FGR). Apparatus and methods disclosed in this application employ improved methods of recovering and controlling boiler stack flue gas and use of novel combustion chamber designs to reduce NOX produced in the combustion process.
In a preferred embodiment, the invention disclosed herein is utilized in a compact boiler. Compact boilers are typified by heat release per cubic foot of boiler occupancy or unit space required for installation of several orders of magnitude greater than conventional boilers such as Scotch Marine or D frame designs. Compact boilers necessarily employ small combustion chambers demonstrating high heat release in the order of 500,000 BTU's per cubic foot of combustion volume. Although providing economical and space conserving boilers and fluid heaters, the compact designs result in inherently high temperatures generated in high heat release combustion system demonstrating percentages of NOX in combustion gases as high as 2000 parts/million (Volume, DRY).
Recent governmental regulations arising from air quality laws have drastically reduced the amount of allowable NOX in boiler flue gas which can be vented to the atmosphere. Therefore, many designs employing various techniques to reduce NOX are currently in use, including flue gas recirculation and so-called staged combustion to lower combustion temperatures and thereby hold NOX emissions in the stack gas to a predetermined level.
A typical compact boiler is disclosed and claimed in U.S. Pat. Nos. 2,735,410, 3,226,038 and 3,282,257 incorporated by reference herein. As disclosed in these references, the compact boiler is fully automatic in its operation and provides steam or heated fluids to widely fluctuating thermal loads. Compact boiler control may be of the type disclosed in U.S. Pat. No. 2,735,410, incorporated by reference above, or a similar method including programmed start-up and firing rates proportional to steam and/or heated liquid delivery rates. The fully automatic load range variation can result in "turn down ratios", i.e. the ratio of fuel admission over the boiler operating load range, as high as 600%.
Compact boilers have been in use for many years and demonstrate good operating records, however, in order to legally operate units of this type, it is necessary to reduce the NOX emissions.
Low NOX burners using flue gas recirculation (FGR) are old in the art and include the following U.S. Pat. Nos.
U.S. Pat. No. 4,995,807
U.S. Pat. No. 5,002,484
U.S. Pat. No. 4,162,140
U.S. Pat. No. 4,776,320
While these systems can be effective in reducing NOX, the concepts and structures disclosed cannot be used in boilers of the compact type since the systems disclosed in these references are utilized in large capacity, large volume boilers operating at essentially constant firing rates. These designs, therefore, do not address the difficulties encountered in applying flue gas recirculation to a compact unit having a 600% variation in fuel rate.
Therefore, it is an object of this invention to provide a low NOX boiler using novel flue gas stack recovery method.
It is an additional object of this invention to provide a burner for a compact boiler wherein NOX concentration in boiler flue gas controls flue gas and combustion air mixing, thereby limiting the NOX content of combustion gases emitted to the atmosphere to a predetermined value over a 600% variation in boiler fuel rate.
It is yet an additional object of this invention to provide apparatus for controlling the NOX content of flue gas emitted to the atmosphere from a compact boiler through utilization of flue gas recirculation and a water wall combustion chamber.
It is yet an additional object of this invention to provide a low NOX combustor for a compact boiler utilizing a staged air burner in a water wall combustion chamber.
Further objects and advantages of the present invention will become apparent as the description proceeds and the features of novelty which characterize the invention will become apparent those skilled in the art.